When capturing images, as with a camera, it is desirable to capture images without unwanted distortion. In general, sources of unwanted distortion may be characterized as equipment errors and user errors. Examples of common equipment errors include inadequate or flawed optical equipment, and undesirable characteristics of the film or other recording media. Using equipment and media of a quality that is suitable for a particular photograph may help mitigate the problems associated with the equipment and the recording medium, but in spite of this, image distortion due to equipment errors may still appear.
Another source of image distortion is user error. Examples of common user errors include poor image processing, and relative motion between the imaging device and the subject of the image. For example, one common problem that significantly degrades the quality of a photograph is the blur that results from camera movement (i.e. shaking) at the time the photograph is taken. This may be difficult to avoid, especially when a slow shutter speed is used, such as in low light conditions, or when a large depth of field is needed and the lens aperture is small. Similarly, if the subject being photographed is moving, use of a slow shutter speed may also result in image blur.
There are currently many image processing techniques that are used to improve the quality, or “correctness,” of a photograph. These techniques are applied to the image either at the time it is captured by a camera, or later when it is post-processed. This is true for both traditional “hardcopy” photographs that are chemically recorded on film, and for digital photographs that are captured as digital data, for example using a charged couple device (CCD) or a CMOS sensor. Also, hardcopy photographs may be scanned and converted into digital data, and are thereby able to benefit from the same digital signal processing techniques as digital photographs.
Commonly used post-processing techniques for digitally correcting blurred images typically involve techniques that seek to increase the sharpness or contrast of the image. This may give the mistaken impression that the blur is remedied. However, in reality, this process causes loss of data from the original image, and also alters the nature of the photograph. Thus, current techniques for increasing the sharpness of an image do not really “correct” the blur that results from relative motion between a camera and a subject being photographed. In fact, the data loss from increasing the sharpness may result in a less accurate image than the original. Therefore, a different method that actually corrects the blur is desirable.
In the prior art, electro-mechanical devices for correcting image blur due to camera motion are built into some high quality lenses, variously called “image stabilization”, “vibration reduction”, or similar names by camera/lens manufacturers. These devices seek to compensate for the camera/lens movement by moving one or more of the lens elements; hence countering the effect of the motion. Adding such a device to a lens typically makes the lens much more expensive, heavier and less sturdy, and may also compromise image quality.
Accordingly, it is desirable to have a technique that corrects for distortion in photographs without adding excessively to the price, robustness or weight of a camera or other imaging device, or adversely affecting image quality.
An additional limitation of current digital imaging devices is that the dynamic range of the image sensors are not adequate to capture both shadows and highlights with detail. As a result, many digital photographs result in washed out highlights or completely dark shadows that are devoid of detail. In traditional film photography these problems are experienced less because most types of film have larger dynamic range compared to digital image sensors.
One remedy in dealing with subjects with large dynamic range has traditionally been the use of graduated filters. These are glass filters that are attached in front of a lens to limit the light coming into the lens from certain areas of the subject. These filters in effect compress the dynamic range of the light coming from the subject. For example, if a scene includes a dark meadow below and a very light sky above, a graduated filter that limits light going through it in the upper part of the image reduces the light intensity for the highlights and “compresses” the light dynamic range of the scene. In this way, both highlights, such as the sky, and the shadows, such as the dark meadow, are captured by the camera with detail. Although graduated filters help in many high-dynamic-range scenes, they are not convenient. A person has to carry along one or more graduated filters for each lens, and adjust the orientation of the graduation by rotating the filter every time a picture is taken.
Accordingly, it is desirable to have a technique that enables digital imaging devices to capture subjects with large dynamic ranges, without requiring use of external graduated filters.